The present invention relates to an electrochromic composition comprising at least one reducing compound and at least two oxidizing compounds, said at least two oxidizing compounds having similar oxydo-reduction potentials. More specifically, said at least two oxidizing compounds are selected from viologen derivatives. Said composition can be used as a variable transmittance medium for the manufacture of an optical article, such as an ophthalmic lens.
Electrochromism is a well-known physical phenomenon which is observed with certain classes of chemical compounds that change reversibly colour when a voltage is applied to them. The material undergoes reversible changes in optical properties by oxidation and reduction. Usually the electrochromic material may be colourless when an electric field is not applied and may be coloured when an electric field is applied.
An electrochromic device, i.e. device containing electrochromic compounds, the absorbance of which depends only on the presence of electric field, can thus have two states, a coloured state or visible light absorbing state (when electrically activated) and a bleached state (in the inactive state). The optical transmission properties of the device depend on the electrochromic compounds.
When preparing an electrochromic composition to be used as transparent media for forming high quality optical articles, in particular high quality ophthalmic lenses, the choice of electrochromic compounds is critical. Indeed, electrochromic compounds need not only to show good electrochromic properties such as high absorption of the visible light in the coloured state, low absorption of visible radiations in the bleached state, fast colouring and fading rates, but should also have long-term stability, in particular in the presence of oxygen, and good solubility in conventional solvents. Gathering all the required properties in one single compound is a real challenge. Many studies have already been conducted for providing electrochromic compounds having the best compromise. For example, viologen compounds have been identified as compounds of particular interest due to their high molar absorption coefficient. Their molar absorption coefficient is indeed higher than other electrochromic compounds, such as ferrocene or dihydrophenazine derivatives, usually used in electrochromic devices.
Another difficulty to face when using electrochromic composition in ophthalmic applications is to meet the consumer demand, which requires a wide range of colours available, and in particular neutral colours (i.e. brown, grey, grey-green . . . ). Such neutral colour can be preferentially chosen with respect of the ISO Standard 1836, which defines the relative visual attenuation coefficient of filters of categories 0, 1, 2 and 3. Other examples of tints defining a neutral colour are given in documents U.S. Pat. No. 6,255,238 and U.S. Pat. No. 5,438,024.
Electrochromic composition having the desired colour can be obtained by mixing different electrochromic compounds. These different electrochromic compounds can be oxidizing electrochromic compounds as well as reducing electrochromic compounds.
However, obtaining the desired colour for an electrochromic composition is an exercise much more complex than simply mixing colours. Indeed, in addition to the numerous requirements that the electrochromic compounds should meet, the challenge of using a combination of different electrochromic compounds lays in the compatibility of said electrochromic compounds with each other. For instance, the absorption coefficient of commonly used electrochromic reducing compounds is generally far lower than electrochromic oxidizing compounds such as viologen compounds. On the other hand, the use of a combination of several electrochromic oxidizing compounds in a single composition further requires that the electrochromic oxidizing compounds have oxydo-reducing potentials close enough so that they can change colour simultaneously when a potential is applied to the composition.
Therefore, there is a need for providing an electrochromic composition having the desired colour in the coloured state or visible light absorbing state, while maintaining good electrochromic properties such as high absorption in the coloured state, low absorption of visible radiations in the bleached state, fast colouring and fading rates and long-term stability, in particular to oxygen or ageing performance.